The ecosystem carbon exchanges in the enclosed rainforest of Biosphere 2, an enclosed apparatus comprised of large synthetic ecosystems, were measured and modeled during the winter of 1995-1996 under different atmospheric CO2 concentrations. On eight separate days, this mesocosm was exposed to various levels of CO2 ranging from about 380 to 820 mu mol mol(-1) daily mean and then sealed 24 hours for continuous measurements of ecosystem CO2 fluxes. Our results indicated that net ecosystem carbon exchange in the mesocosm was enhanced by increasing CO2 over the short periods studied (2-7 weeks), but, as expected from physiological studies, the response is not linear. The main effect of shortterm CO2 change was the enhancement of canopy CO2 assimilation, while soil respiration was not affected by the atmospheric CO2 concentration. The whole ecosystem radiation use efficiency was significantly higher under higher CO2. The results of direct measurements were predicted well by a simple canopy model (the 'big-leaf' model) that incorporates current physiological understanding of the biochemistry of leaf photosynthesis. Validation of this model with a range of CO2 and light levels indicates that it can be used with confidence to predict the responses of natural ecosystems to global climate change. Response of ecosystem processes to elevated CO2 with relaxation time longer than a few weeks could not be resolved in this study, but longer-term closure experiments are planned to examine these processes.